Luminaires are employed in a number of lighting applications, such as ambient or space lighting, accent lighting, wall washing, signage, advertising, decorative and display lighting, façade lighting, and custom lighting. Luminaires typically include a number of high-brightness incandescent, fluorescent, neon, or light-emitting diode (LEDs) type light sources coupled to a power management system for supply of energy and control of the desired utility.
A general drawback of high-brightness light sources includes the release of excessive quantities of heat under operating conditions. While being relatively efficient certain LEDs offer high energy densities and generate large amounts of waste heat in small spaces. The use of high-brightness LEDs in illumination applications usually requires some form of temperature control to mitigate the risks of catastrophic failure modes of the LEDs and other components of the luminaire.
Temperature control of LEDs can entail maintaining sub-optimal operating conditions below nominal power ratings or, alternatively, improving the rate at which heat can dissipate from the LED or another heat source to a cooler environment. There are a number of solutions known in the art including active and passive cooling including heat sinks and heat pipes. Heat sinks comprise heat-conductive elements that can be thermally coupled to a heat source. The heat sink needs to be thermally coupled to the heat source and the environment; the coupling between the heat sink and the heat source is typically of a conductive nature and the coupling between the heat sink and the environment is typically convective in nature. Heat sinks provide large surfaces to improve cooling efficiency via thermal convection in a proximate space of the environment. Heat sinks typically include a large number of structural cooling elements such as fins, pins or posts to increase the surface area between the heat sink and the environment. Forced convection via fans, for example, can be employed to improve convection and increase the efficiency of the heat sink.
Heat sinks are widely employed for the thermal management of luminaries but their form and use have limited applicability for the direct cooling of LEDs as interferences with the light emission of the LEDs are usually undesired. Heat sinks provide limited heat dispersion capabilities and occupy relatively large spaces in order to work effectively. Heat sinks can only be employed where they can be adequately thermally coupled to LEDs or other heat sources and may therefore be excluded from use in systems with high device integration densities.
A heat pipe is another type of thermal management device. Heat pipes comprise a thermally conductive body in which a certain amount of a heat transfer medium such as a gas, liquid or other fluid is hermetically contained. Heat pipes are intended to rapidly transfer heat from one end to another end of the heat pipe while being relatively small. One end of the heat pipe is thermally coupled to a heat source and the other end can be thermally coupled to a device of lower temperature. One end of the heat pipe typically absorbs thermal energy generated by the heat source, initiating the temperature inside the heat pipe to rise which can cause the heat transfer medium inside the heat pipe to undergo a phase transition, for example the heat transfer medium may evaporate. As a result, the absorbed heat from the heat source provides the energy to overcome the latent heat of the phase transition of the heat transfer medium which in return provides an effective cooling mechanism. Typically the heat transfer medium evaporates and diffuses or buoys away from the heat source, through one or more cavities within the heat pipe to reach a cooler end of the heat pipe where it condenses. Transport of the condensate back to the hot end of the heat pipe is usually either gravitational or aided by capillary effects. Enhanced capillary effects can originate from additional elements which can be disposed inside certain types the heat pipes for example a wicking structure. The condenser end of the heat pipe can be cooled via coupling to a heat sink, for example.
Many known heat pipe designs suffer from a number of deficiencies. Effective heat pipes cannot be built arbitrarily small and often require additional elements such as heat transfer plates when used for cooling relatively small devices in highly integrated systems. Generally, the integration of heat pipe cooling technology in tightly-packaged luminaries can be difficult. Additionally, the charging of heat pipes with cooling media may require controlled pressure conditions during manufacturing. It is difficult in standard heat pipe integration designs to mold heat pipes along with injection molded parts of the luminaire such as optical elements, for example.
Furthermore, every additional component included in the thermal management system increases the complexity of the system design, decreases cost effectiveness and also introduces additional interfaces which can act as a heat flow barrier which can significantly reduce the overall cooling efficiency. For example, LED dies can be mounted on a substrate which can be thermally coupled to a heat spreader plate which itself in return can be thermally coupled to a heat pipe and so forth. Each of these elements needs to be in intimate thermal contact with its adjacent element for the cooling system to work effectively.
There is therefore a need for a thermal management system that offers improved heat transfer efficiency and which can be integrated with optical elements and suitable for use in LED-based luminaries.
This background information is provided to reveal information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.